Justin Bieber -- The Comedy Central Roast is Therapy

Justin Bieber ASKED to be skewered by Comedy Central because he wants to make a public declaration he's screwed up a lot in the past, but he's figured it out and has changed for the better.

TMZ first reported ... Justin himself asked to be roasted to celebrate his 21st birthday. We're told he views it as a form of therapy ... acknowledging hijinks from DUIs to pissing in kitchen buckets.

But here's the thing ... Justin definitely seems to have changed over the last 4 months. People around L.A. tell us he's polite again, respectful and extremely generous. He went to Mozza in L.A. Sunday night but the restaurant was full. Two guys saw him and gladly moved to the bar so he could have a table. We're told Justin walked up to thank them and then bought their dinner.

And Justin was in line at a restaurant a few days back and randomly bought a meal for some people behind him.

People who have regular contact with Bieber tell us they've seen a radical change. As one put it, "The person I knew is back."

As for why the roast will be therapy ... we're told Justin thinks the evening will end the chapter of his life where he messed up. He'll take his lumps, and then move on as a real adult.